PFBC power plant

ABSTRACT

A PFBC power plant with a combustor (3) enclosed within a pressure vessel (1). The combustor (3) is formed as a polygonal prism with, for example, hexagonal cross section. The bed section (3a) of the combustor (3) accomodates a steam generator constructed from at least three groups (23) of plane tube discs (24), in which groups (23) the discs are oriented parallel to opposite sides walls in the combustor (3). Each tube disc (24) may comprise both evaporation and superheater tubes (25a, 25b, 25c) which may comprise tubes in several parallel planes. In a preferred embodiment, each group (23) of tube discs (24) fills up a parallelepipedic space in the lower part (3a) of the combustor (3).

TECHNICAL FIELD

The invention relates to a PFBC power plant. PFBC are the initialletters of the English expression Pressurized Fluidized Bed Combustion.In a PFBC power plant the combustion is performed in a fluidized bed ofparticulate material, usually mainly consisting of limestone or dolomitewhich acts as sulphur absorbent. The combustion takes place at apressure which considerably exceeds the atmospheric pressure. Thecombustor is suitably enclosed within a pressure vessel and issurrounded by compressed combustion air. The combustion gases areutilized by a gas turbine which drives a compressor, which compressesthe combustion air, and a generator. The bed section of the combustorincludes tube coils which absorb heat from the bed, cool the bed andgenerate and superheat steam for a steam turbine which drives agenerator.

BACKGROUND ART

Hitherto proposed and designed commercial PFBC power plants arerelatively small and have a power of up to about 200 MWe. Rectangularcombustors have been used to obtain a geometrically simple constructionof the tube system in the bed section of the combustor. A cleaning planein the form of groups of cyclones for separation of dust from thecombustion gases has been provided between the combustor and thesurrounding pressure vessel.

The problem involved is that the hitherto proposed and designed PFBCpower plants require a pressure vessel with a relatively large diameterin view of the geometrical shape of the combustor. In the firstgeneration PFBC power plants, the advantage of the simple tube laying inrectangular combustors has made up for the additional cost of a largerpressure vessel. When doubling or multiplying the power it is importantto increase the ratio between the cross-section areas of the combustorand the pressure vessel.

SUMMARY OF THE INVENTION

According to the invention, the combustor is made as a polygonal prismwith at least six side walls. Most suitably, the combustor is made withhexagonal cross section with a bed section with a steam generatorconsisting of evaporation and superheating tubes, and a freeboardsection for reception of the combustion gases from the bed. At its top,the freeboard is connected to at least one substantially vertical ductwhich conducts the combustion gases to a cleaning plant which is locatedin a space formed between the duct and the surrounding pressure vessel.

The cleaning plant comprises a number of groups of cyclones connected inseries, hot gas filters, or a combination of cyclones and hot gasfilters.

The combustor design and the location of the gas cleaning plant permit aconsiderably better ratio between combustor cross section and pressurevessel cross section. An improvement of the order of magnitude of 20% ormore is possible. The reduction of the pressure vessel diameter and thenecessary wall thickness of the pressure vessel entails a considerablereduction of weight and cost. Further, the shape of the combustorpermits a simpler suspension. Suitably, the combustor is suspended, atits corners, from rods which are attached directly to the pressurevessel or to relatively short beams in the pressure vessel. Anadditional advantage with the embodiment is that the plane walls of thecombustor will be shorter. This reduces the length of surrounding beamswhich absorb forces caused by the pressure difference between the insideand outside of the combustor. The weight and cost of the frames arereduced.

The vertical duct between the freeboard of the combustor and thecleaning plant entails a favourable outflow and mixture of thecombustion gases and combustion of accompanying unburnt fuel. Further,separation of coarse particles may take place in the duct by means ofsimple separating devices. NOX reduction may take place in the duct bythe injection of additives, for example ammonia. The duct may also beutilized as a secondary combustion chamber to increase the gastemperature and hence the gas turbine power.

According to a preferred embodiment of the invention, plane verticaltube discs are arranged in a combustor of hexagonal cross section inthree groups with the tube discs in the respective group orientedparallel to two opposite side walls. In one embodiment the combustor maybe designed with a hexagonal annular bed section. In another embodimentthe tube discs fill up the entire hexagonal space and each of the tubedisc groups fills up a parallelepipedic space.

The tube discs are oriented parallel to the side walls in the combustor.Despite the shape of the combustor, simple plane tube discs of equalsize can be used in the entire combustor.

In one embodiment of the invention, the tube discs are constructed fromtubes in three parallel, adjacent planes. Of the tubes of the differentplanes in a tube disc, at least one tube plane is included in anevaporator and one or two tube planes in a superheater. In one disc, thetubes in a central tube plane may be suspend from supporting members inthe combustor and support the other tube planes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to theaccompanying drawings.

FIG. 1 shows a view and a section of the pressure vessel of a PFBC powerplant with a combustor, a cleaning plant, and so on. FIG. 2schematically shows a horizontal section through the pressure vessel andthe combustor at II--II in FIG. 1. FIG. 3 shows a vertical section ofthe combustor at III--III in FIG. 2. FIG. 4 shows a perspective view ofa parallelepipedic group of tube discs and collecting pipes forfeedwater and steam. FIG. 6 schematically shows two tube discsrepresentative of the whole steam generator and the flow through these.FIG. 5 schematically shows an end view of a group of three tube discswhich are supported by tubes in the central tube plane of the respectivetube disc. FIG. 7 schematically shows an alternative way of arrangingtube discs in a hexagonal combustor. FIG. 8 shows an alternative way ofarranging tubes in a tube discs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the figures, 1 designates a cylindrical pressure vessel whichencloses a combustor 3, a container 5 for receiving and storing bedmaterial, a cleaning plant 7 consisting of a number of groups ofseries-connected cyclones 7a, 7b, 7c as well as certain other auxiliaryequipment. The lower part of the combustor 3 forms the bed section 3a ofthe combustor 3. This section accomodates heat-absorbing tubes whichform a steam generator. The upper part of the combustor 3 forms thefreeboard 3b of the combustor 3 which receives combustion gases leavingthe bed of the combustor 3. The free-board 3b of the combustor isconnected to a duct 9 which connects the combustor 3 to the cleaningplant 7, which is located in the annular space 11 between the duct 9 andthe surrounding pressure vessel 1. Cleaned gases are collected in theconduit 13 and are passed to a gas turbine included in the power plant.Separated dust is removed through a pressure-reducing cyclone ash cooler15. As shown in FIGS. 2 and 7, the combustor 3 in a preferred embodimenthas a hexagonal cross section and forms a prism. At its corners 17 thecombustor 3 is suitably suspended from pendulums 19 which may beattached to brackets or beams 21 fixed to the pressure vessel.

In the embodiment shown in FIGS. 2 and 4, the tube system 23 in the bedsection 3a of the combustor 3 is divided into three groups 23a, 23b, 23cwith a rhombic cross section, each of which fills up a parallelepipedicspace in the bed section 3a. By this arrangement, a possibility isprovided of completely filling up the cross section in the combustor 3with plane parallel tube discs 24.

In the embodiment shown, each tube disc group 23 is constructed fromtube discs 24 consisting of tube coils 25a, 25c, 25c with differentfunctions and comprises tubes included in the evaporator (EVA),superheater I (SHI), superheater II (SHII) and intermediate superheater(ISH) of the steam generator. The composition within different tubediscs 24 may vary to obtain a suitably adapted heat-transferring surfacein the evaporator section and the superheater sections of the steamgenerator in view of the current performance requirements. The schematicFIG. 4 shows tube discs 24 with three tube coils 25a, 25b, 25c in eachtube disc in one of the parallelepipedic spaces of the combustor 3. Insuch a space there may in reality be 20-40 discs 24. As will be clearfrom FIGS. 3, 4 and 5, the tube discs 24 are suspended from the centraltube coil 25b. This is suspended at its mid-point from beams 71 in oneof the outer walls 3c of the combustor 3 and in a wall 3d between theparallelepipedic spaces inside the combustor 3. The beams 71 areconnected to the conical ceiling of the combustor 3 by cooled connectingrods (tubes) 73 and 75.

In the embodiment of the steam generator shown in diagrammatic form inFIG. 6 there are two types of tube discs 24a and 24b. The disc 24aincludes tubes 27 with two parallel tube coils 27a, 27b which are partof the evaporator EVA of the steam generator, a tube coil 29 which ispart of a first superheater SHI and tubes 31 with two parallel tubecoils 31a, 31b which are part of an intermediate superheater ISH. Thetube discs 24b include tubes 29 which are part of a first superheaterSHI, tubes 33 in two tube coils 33a, 33b which are part of a secondsuperheater SHII, and tubes 31 in two parallel tube coils 31a, 31b whichare part of an intermediate superheater ISH. The superheater tubes 29 inthe first superheater SHI serve as supporting tubes. In the planes ofthese tubes 29 and between the vertical parts thereof, other superheatertubes may be positioned (in this case the intermediate superheater tubes31 are shown in this position).

Feedwater from the feedwater container 35 is pumped by means of thefeedwater pump 37 to the distributing pipes 39 of the evaporator andfurther to the evaporator tubes 27, is collected after passage throughthe evaporator in the collecting pipes 41 of the evaporator and ispassed to the steam separator 43. Separated water is returned to thefeedwater container 35 through the conduit 45 and the water levelregulating valve 47 to the feedwater container 35. The steam is passedvia the distributing pipes 46 of the first superheater SHI to the tubes29 in the first superheater SHI and is collected after passage throughthe first superheater SHI in the collecting pipes 51 of the firstsuperheater SHI, passes through the steam cooler 54 where thetemperature of the steam is regulated by means of water injection beforeit is supplied via the distributing pipes 52 of the second superheaterSHII to the tubes 33a, 33b of the second superheater SHII. After passagethrough the superheater SHII, the superheated steam is collected in thecollecting pipes 53 of the second superheater SHII and is passed throughthe conduit 55 to the high-pressure section 57a of the turbine 57. Thesteam from this turbine section is passed in a conduit 59, via thedistributing pipes 61 of the intermediate superheater ISH, to the tubes31a, 31b of the intermediate superheater ISH, is collected after passagethrough the intermediate superheater ISH in the collecting pipes 63 ofthe intermediate superheater ISH and is returned to the intermediate andlow-pressure section 57b of the turbine 57 and from there to a condenser(not shown) via the conduit 67.

As shown in FIG. 3, the combustor may be equipped with, for example,gas-fired burners 77 in the freeboard section 3b or in the duct 9 toincrease the gas temperature, especially in case of partial load. Theduct 9 may include devices 79 with nozzles 81 for injection of aNOX-reducing substance, for example ammonia in the flue gases.

FIG. 7 shows an alternative way of arranging the tube discs 24 in alarge combustor. In this arrangement, the size of the tube discs 24 isreduced. In the centrally formed space 83, auxiliary equipment may bearranged.

We claim:
 1. A PFBC power plant comprising a combustor enclosed within asubstantially cylindrical pressure vessel of circular cross section andwhereinthe cumbustor comprises one bed section and one freeboardsection, the power plant also comprising a steam generator consisting ofevaporation tubes and superheating tubes, and a cleaning plant for fluegases is connected to the freeboard section,wherein the combustor isformed as a polygonal prism with six side walls, the bed section ofwhich comprises a fluidized bed which is common to the whole combustor,evaporation and superheating tubes arranged in three groups are placedin the fluidized bed, and the freeboard section comprises a freeboardwhich is common to the whole combustor and which receives flue gasesfrom the fluidized bed.
 2. A PFBC power plant according to claim 1,wherein evaporation and superheating tubes are designed as a number ofparallel plane discs in three groups and with the discs in therespective group oriented parallel to two opposite sides in thehexagonal combustor and wherein each tube disc comprises tubes whichconstitute evaporation tubes and superheating tubes.
 3. A PFBC powerplant according to claim 2, wherein each group of tube discs fills up aparallelepipedic subvolume of the bed section.
 4. A PFBC power plantaccording to claim 1, wherein each tube disc consists of tubes inseveral, preferably three planes and wherein the tube disc is suspendedfrom the combustor in the central tube plane.
 5. A PFBC power plantaccording to claim 1, wherein the top of the combustor is connected toat least one duct which connects the freeboard section to the cleaningplant.
 6. A PFBC power plant according to claim 5, wherein the cleaningplant is housed above the combustor in the space between the duct/ductsand the walls of the pressure vessel.
 7. A PFBC power plant according toclaim 6, wherein the cleaning plant comprises cyclones and hot gasfilters and is arranged in an annular space surrounding a central duct.8. A PFBC power plant according to claim 1 wherein the combustor isformed with a hexagonal annular bed section which surrounds an internalhexagonal space containing auxiliary equipment.
 9. A PFBC power plantaccording to claim 1, wherein the combustor is suspended from supportingmembers which are attached to the pressure vessel wall and to thecorners of the combustor.
 10. A PFBC power plant according to claim 2,wherein the combustor is formed with a hexagonal annular bed sectionwhich surrounds an internal hexagonal space containing auxiliaryequipment.
 11. A PFBC power plant according to claim 6, wherein thecleaning plant comprises hot gas filters and is arranged in an annularspace surrounding a central duct.